1. Field of the Invention
The present invention relates to an optical waveguide provided with an optical waveguide path through the inside of which a light signal is propagated and a method for producing the same, more particularly relates to a optical waveguide suitable for transmitting a light signal in an ultra high speed signal processing circuit or parallel type digital signal processing circuit or other signal processing circuit, optical communication, and optical connection of an optical link or optical fiber channel etc. to an optical module and a method for producing the same.
2. Description of the Related Art
In recent years, due to the dramatic improvements in wireless telecommunications technology used for cellular telephones and the like, wired telecommunications technology used in integrated service digital networks (ISDN) and the like, and the processing capabilities of personal computers (PC) and other data processing devices and to the digitalization of audiovisual (AV) apparatuses and so on, there has been an ongoing movement toward transferring all kinds of media via data communications networks. Further, use of the Internet, first and foremost, and local area networks (LAN) and wide area networks (WAN) and other such data communications networks is now spreading in business and among individuals. Due to this, an environment will be realized in the future in which household electrical appliances and AV equipment are connected in the home through a PC to form a network in which information will be able to be freely transferred through a telephone line, CATV (cable television or community antenna television) line, ground wave television channel, or satellite broadcast or satellite communication channel or other data transmitting means.
In order to freely transfer image data handled at a high speed of several Mbps to 10 or more Mbps by such a data transmitting means, a data transmission rate of for example about 10 Mbps to 1 Gbps is demanded. Optical communications and transmission technology can realize such data transmission rate. For example, in a so-called trunk system telecommunications network extending over 10 km to 100 km such as an optical cable laid on the bottom of the sea, wide use is being made of optical communications and transmission technology due to their low loss and economy.
On the other hand, as the means for transmitting data over a relatively short distance, for example, between boards in an apparatus or between chips on a board, use has mainly been made of wired communications and transmitting means such as twisted pair cables or coaxial cables. Recently, although optical fiber channels and optical data links and other technology for using optical transmission have begun spreading in use, at the present time they have not yet spread to an extent of the optical communications and transmitting means replacing wired communications and transmitting means. One of the reasons for this is the issue of cost versus effect. Specifically, for example, there can be mentioned the points that, in order to maintain the performance of optical communications (for example, transmission rate and transmission quality), technology is required for precise positioning between a light emitting element and a light receiving element constituting the optical communications device and the optical fiber, that countermeasures for light leakage, consideration of electromagnetic interference, and countermeasures to noise are necessary, and that, as a result, the device becomes complex and expensive etc.
On the other hand, advances in the technology for integrated circuits (IC) and large scale integrated circuits (LSI) have led to improvements in operating rates and scales of integration. For example, rapid improvements are being made in the performance of microprocessors and the capacity of memory chips. Further, amounts of data being handled by PCs connected by networks are rapidly growing as well. Accordingly, the issues have arisen of how to deal with the rise in the frequency of the signal processing clocks, the rise in the degree of parallel operation, and the rise in the speed of the access to the memories. In view of this situation, semiconductor chips are being miniaturized and the gate lengths of the transistors reduced along with this and the driving capabilities being improved so as to increase the operating rate inside semiconductor chips. In memory access circuits and processors of multi-microprocessor (MPU) configurations, however, the parasitic capacitance component of the portion which becomes necessary at the time of mounting, for example, the package of the semiconductor device, is large, so high speed signal transmission operation becomes difficult in the electric wiring connected to the outside of the semiconductor chips. Further, application of a high speed signal to the electric wiring can cause spikes in the current or the voltage and can cause electromagnetic interference (EMI) and other types of electromagnetic interference noise, reflection noise, and crosstalk noise.
Therefore, in order to transmit high speed signals even between semiconductor chips on a circuit board and other short distance signal transmission routes, for example, it is considered desirable that the signals be transmitted by light and that, in particular, use be made of an optical transmission and communications system using an optical waveguide path as the transmission path. When transmitting a signal by light, the signal delay due to the “CR” time constant (C: capacitance of the wiring, R: resistance of the wiring) of the wiring can be eliminated and the influence of electromagnetic noise can be avoided, therefore transfer of high speed signals becomes possible. Accordingly, it is necessary to maintain the communications performance of optical communications and transmission equal to the communications performance of wired communications and transmission, and realize a reduction of the costs in order to promote the use of short distance optical communications and transmission systems even in the field of equipment for general users.
In order to maintain the communications performance of optical communications and transmission equal to the communications performance of wired communications and transmission, it is required that for example the light propagation loss of the optical waveguide path be kept small. As a material having a small light propagation loss satisfying this condition, there is a quartz-based material. Quartz has an extremely good light transmission property as has already been proved by optical fibers. When an optical waveguide path is prepared by quartz, a reduction of the loss to less than 0.1 dB/cm is achieved.
FIG. 32 is a view of an example of the configuration of an optical waveguide of the related art (see Japanese Unexamined Patent Publication (Kokai) No. 62-204208). This optical waveguide is comprised of a flat silicon substrate 501 on which are formed thin film multilayer wiring 505 with the wiring insulated from each other by an insulating layer 506 and on which are provided an optical waveguide path 502 made of quartz and an LSI 504. Further, above each end region of the optical waveguide path 502, a light receiving element 503 and a light emitting element (not illustrated) are formed and electrically connected to the LSI 504 arranged in their vicinity. In this optical waveguide, the light signal emitted from the not illustrated light emitting element is propagated inside the optical waveguide path 502, reflected at an end surface 502a, and made to strike the light receiving element 503.
As the method for producing an optical waveguide having such a configuration, there is known the method of forming the optical waveguide path 502 on the silicon substrate 501 on which the thin film multilayer wiring 505 are formed, then using for example reactive ion etching (RIE) or other anisotropic etching to process the end surface 502a of the optical waveguide path 502 so as to become approximately 45° with respect to the surface of the silicon substrate 501 and then further mounting the light receiving element 503, light emitting element, and LSI 504 on the silicon substrate 501.
Summarizing the disadvantage, in the method for producing the optical waveguide mentioned above, however, since the optical waveguide path 502 made of quartz is supposed to be formed on the silicon substrate 501, thin film technology had to be used for forming the optical waveguide path 502. Formation of the optical waveguide path 502 using thin film technology results in an excellent dimensional accuracy, but conversely has the disadvantage that the formation and processing of a film having a thickness of no more than several μm are difficult.
Further, even if a signal can be transmitted at a high speed by light, the electric power has to continue to be supplied and various control signals of relatively low speed etc. have to continue to be transmitted by electrical wiring (electrical signals). For this reason, it suffers from the disadvantage that while it is essential to form the thin film multilayer wiring 505 as the electrical wiring on the silicon substrate 501, this electrical wiring forming region would be too costly and poor in practicality if being the usual circuit board size (several 10s of cm square) or module size (several cm square).
In order to overcome the disadvantage, it can be considered to form the optical waveguide path on a printed circuit board on which the electric components can be mounted. The surface of a circuit board fabricated by such a thick film process, however, has thick films of metal formed by for example a plating process and therefore has a large unevenness. For this reason, if the optical waveguide path is formed on such a printed circuit board, the disadvantage arises that the uneven shape of the surface of the board will end up having an effect on the shape of the optical waveguide path and end up leading to an increase of the light propagation loss of the optical waveguide path and a reduction of the dimensional accuracy.
Further, in the wet etching or washing etc. when forming the optical waveguide path on a circuit board, a step of immersing the entire board in acidic and alkaline solutions or organic solvents or the like becomes necessary, therefore there is the problem of the board being liable to be damaged. Further, the board is also liable to be damaged at the time of dry etching or the time of the high temperature heat treatment. Accordingly, it is difficult to use a circuit board formed by the thick film process as the substrate, so it was necessary to use an expensive substrate having a high heat resistance and other characteristics.